1. Field of the Invention
The present invention relates generally to communication systems, and in particular, relates to polarization tracking in wireless optical communication systems.
2. Background Information
With the increasing popularity of wide area networks, such as the Internet and/or World Wide Web, network growth and traffic have exploded in recent years. Network users continue to demand faster networks, and as network demands continue to increase, existing network infrastructures and technologies are reaching their limits.
An alternative to existing hardwire or fiber network solutions is the use of wireless optical telecommunications technology. Wireless optical telecommunications utilize beams of light, such as lasers, as optical communications signals, and therefore do not require the routing of cables or fibers between locations. Data or information is encoded into a beam of light, and then transmitted through free space from a transmitter to a receiver.
For point-to-point free space laser communications, the use of narrow optical beams provides several advantages, including data security, high customer density, and high directivity. High directivity makes the achievement of high data rates and high link availability easier, due to higher signal levels at a receiver. In order to take full advantage of this directivity, some form of tracking is often necessary to keep the antennas of a transmitter and of the receiver properly pointed at each other. For example, a transmitted optical beam with a 1-mrad divergence has a spot diameter at the receiver of about 1 meter at a 1 km range. Thus, movement of the transmitter or receiver by even a small fraction of the divergence (or field-of-view) could compromise the link unless active tracking is employed. Since high-speed communication channels utilize extremely sensitive detectors, such systems require equally sensitive tracking systems.
Charge coupled device (CCD) arrays or quadrant cell optical detectors (sometimes referred to as xe2x80x9cquad cellsxe2x80x9d) may be used as tracking detectors in a tracking system. In either case, an electrically controllable steering mirror, gimbal, or other steering device may be used to maximize an optical signal (e.g., light) directed at a high speed detector, based on information provided by the tracking detector. This is possible since optical paths for tracking and communication are pre-aligned, and the nature of a tracking signal for a perfectly aligned system is known. CCD tracking is very sensitive, offers potentially more immunity to solar glint because of the ability to ignore glint xe2x80x9cfeaturesxe2x80x9d on the CCD array, and is in general a well-proven tracking method. However, at certain wavelengths, a lower wavelength tracking beam is often necessary due to limitations of CCD detection systems.
In the case of quad cells, for an aligned optical system, an equal signal in all four quadrants will normally indicate that the steering mirror has optimally directed the optical communication signal onto the high-speed detectorxe2x80x94if there is any deviation from this, the steering mirror will direct the optical signal back to this optimum equilibrium.
The signal on the quad cells may be direct current (DC). DC tracking may use the average signal content of the communications channel from each quad cell. A problem with DC is that quad cell electronics cannot distinguish between an actual optical signal and a signal that may have come from solar background radiation or from imperfect transmit/receive isolation. Thus, the tracking system may misalign the transmitter and receiver in the presence of background light.
Collectively, these tracking methods often suffer from disadvantages including bulkiness, expense, complexity, and inaccuracy or imperfect performance. Accordingly, improvements are needed for tracking in wireless optical communication systems.
According to one aspect of the invention, a method is provided that includes modulating a polarization of a light signal, and transmitting the light signal having the modulated polarization. The light signal having the modulated polarization is received, and an amplitude-modulated light signal based on the modulated polarization is generated. The amplitude-modulated light signal is used to perform tracking of a receiver with respect to the received light signal.